The present invention relates to a solid polymer electrolyte fuel cell featuring high cell performance, compact size and low cost, wherein the fuel cell comprises a unit cell structure including a solid polymer electrolyte film as an electrolyte retaining layer and electrodes on both major surfaces thereof. Reactant gases, i.e., fuel and oxidant gases, are supplied to the electrolyte and electrodes to obtain electrical energy by electrochemical reactions. More specifically, the present invention relates to a flow path configuration for the reactant gases and coolant, whereby condensation and liquefaction of moisture in the reactant gases is prevented and uniform cooling of the unit cell is achieved.
FIG. 4 is a cross-sectional view schematically showing the basic unit cell structure of a conventional solid polymer electrolyte fuel cell. As shown in FIG. 4, laminated anode and cathode electrodes 14 and 16, respectively, are arranged to contact with the respective surfaces of a solid polymer electrolyte film 15. Separators 10 and 11, each consisting of a gas-impermeable metal plate, are arranged on the respective outer surfaces of the electrodes 14 and 16. Seal members 17, preferably elastic seal members, are arranged on the side end portions between the separators 10 and 11 and the electrolyte film 15 to keep the electrodes 14 and 16 and the electrolyte film 15 in an air-tight environment.
So arranged, electrolyte film 15, electrodes 14 and 16, separators 10 and 11 and the seal members 17 constitute the unit cell structure of a conventional solid polymer electrolyte fuel cell. The separator 10 is cyclically bent such that an oxidant gas conduit 7 is formed between the separator 10 and the cathode electrode 16. The oxidant gas conduit 7 is used for conducting the flow of oxidant gas within the unit cell and for expelling excess oxidant gas from the unit cell.
The separator 11 is also cyclically bent such that a fuel gas conduit 8 is formed between the separator 11 and the anode electrode 14. The fuel gas conduit 8 is used for conducting the flow of fuel gas within the unit cell, and for expelling excess fuel gas from the unit cell. A coolant for maintaining the fuel cell at a certain temperature flows through a coolant conduit 18 formed between the adjoining separators 10 and 11.
FIG. 5 is a plan view of a conventional separator structure, viewed from the side of the cathode electrode 16 in FIG. 4. As shown in FIG. 5, an oxidant gas inlet 1 and an oxidant gas outlet 2 extend vertically in the direction of lamination, i.e., along the normal to the illustrated plane. A fuel gas inlet 3 and a fuel gas outlet 4 extend vertically in the direction of lamination. A coolant inlet 5 and a coolant outlet 6 also extend vertically in the direction of lamination. The oxidant gas provided from the oxidant gas inlet 1 flows through the oxidant gas conduit 7 winding so that the oxidant gas may be delivered uniformly over the electrode to contribute to the electrochemical reactions. The excess oxidant gas is expelled through the oxidant gas outlet 2.
The solid polymer electrolyte film 15 conventionally comprises a cation conductive film consisting of a cation exchange film of the polystyrene family that includes sulfonic acid groups, a mixture film of fluorocarbon sulfonic acid and polyvinylidene fluoride, a fluorocarbon matrix to which trifluoroethylene is graft-copolymerized, and a perfluorocarbon sulfonic acid film (Nafion Film from Du Pont Co.). The solid polymer electrolyte film 15 includes proton (hydrogen ion) exchange groups in the molecule, and exhibits a resistivity of 20 .OMEGA.-cm.sup.2 or less at the ordinary temperature when saturated with water, and also functions as a proton-conductive electrolyte. The saturated water content in the film changes reversibly with temperature.
In FIG. 4, the anode and cathode electrodes 14 and 16, respectively, include a catalyst layer containing catalytic material and a electrode base. The electrode base is used for retaining the catalyst layer, for conveying the reactant gases, and for generating an electric current. A three phase boundary is formed between the catalyst layer and the solid polymer electrolyte film 15 by: arranging the catalyst layer in close contact with the solid polymer electrolyte film 15; supplying the fuel gas, including hydrogen, to the anode side; and supplying the oxidizing gas, including oxygen, to the cathode side. Accordingly, electric energy is produced in accordance with electrochemical reactions (1) and (2), through which water is yielded:
At the anode EQU H.sub.2 .fwdarw.2H.sup.+ +2e.sup.- ( 1) PA1 At the cathode EQU 2H.sup.+ +(1/2)O.sub.2 +2e.sup.- .fwdarw.H.sub.2 O (2)
The catalyst layer generally includes platinum catalyst small grains, a water repellent fluororesin and fine bores for efficiently diffusing the reactant gases to the three phase boundary. The catalyst layer must form a adequate three phase boundary.
The above-described unit cell structure of FIG. 4 is limited to output voltages of 1 V or less. In order to provide a fuel cell that generates as high an output voltage as practical, many unit cells are laminated together. Typically, the solid polymer electrolyte fuel cell is operated at 50 to 100.degree. C. to lower the resistivity of the film and maintain a high power generating efficiency for the fuel cell.
Since the solid polymer electrolyte film 15 functions as the proton-conductive electrolyte when the resistivity of the solid polymer electrolyte is lowered by saturating the solid polymer electrolyte film 15 (as the electrotype retaining layer) with water, the water content of the solid polymer electrolyte film 15 must be maintained at the saturation level in order to maintain a high power generating efficiency. Therefore, water is supplied to the reactant gases to raise the humidity of the reactant gases. The humidified reactant gases are supplied to the fuel cell thereby suppressing water vaporization from the film and preventing the solid polymer electrolyte film 15 from drying.
The aforementioned water yielded in connection with power generation, as described above, flows through the gas conduits along with excess reactant gases. The water is carried away by the gas conduits and drained externally. Consequently, the water content of the reactant gases has a distribution in the gas flow direction, i.e., the water content of the reactant gases increases in the downstream direction. Therefore, when the gases saturated with water are supplied to the fuel cell, supersaturated moisture is contained in the gases on the outlet side. The supersaturated moisture condenses to form water drops, which in turn prevent diffusion of the reactant gases to the reactive portions of the electrodes.
The resulting condensation consequently lowers the efficiency of the electrochemical reactions within the cell. It is therefore desirable to expel the excess moisture without generating excess condensation in the fuel cell.
FIG. 6(a) is a diagram illustrating the flow of coolant, and FIG. 6(b) is a graph showing the cell temperature distribution in the direction of coolant flow in FIG. 6(a). Conventionally, cell structure is such that reactant gases and coolant flow in the same direction. Due to the heat generated during the electrochemical reactions, the reactant gas temperatures increase along the downstream direction of the fuel cell laminate, as shown in FIG. 6(b). The higher gas temperature on the downstream side serves to retard condensation of the water vapor in the reactant gases. This is desirable in conventional solid polymer electrolyte fuel cells because the power generating efficiency thereby is maintained by moisturizing the reactant gases to saturation, as the unwanted condensation of moisture within the cell would impair the efficiency of the electrochemical reactions.
However, conventional solid polymer electrolyte fuel cells are also characterized by lower cooling efficiency in the downstream direction. As shown in FIG. 6(b), the cell temperature increases in the downstream direction but drops in the vicinity of the outlets due to heat radiation from the end face of the unit cell to air. Therefore, due to lower temperatures near the reactant gas outlets, moisture in the reactant gases condenses in the vicinity of such outlets, thus lowering the power generating efficiency of the unit cell.
Also, in a conventional unit cell structure utilizing the metal separators 10 and 11 as shown in FIG. 4, uniform cooling of the unit cell is difficult to accomplish due to the non-uniformity of the coolant flow paths. This may be further appreciated from FIG. 7, which is a plan view of separators (10, 11) exemplifying the flow path of the coolant in a conventional solid polymer electrolyte fuel cell. As there shown, the coolant flow path is formed by recessed portions (hatched portions) formed on the back surface of the separators (10, 11) in correspondence with the oxidant conduit 7. Coolant flows via the recessed (hatched) portions from the coolant inlet 5 to the coolant outlet 6 in the direction of fuel gas flow, i.e., from the fuel gas inlet 3 to the fuel gas outlet 4. However, as shown in FIG. 7, the coolant flow path is non-uniform and is characterized by many "blind alleys," which contribute to the non-uniform cooling of the unit cell.
FIGS. 8 and 9 are perspective views showing two different combinations of the adjoining separators 10 and 11 in a conventional solid polymer electrolyte fuel cell. In FIG. 8, the oxidant gas conduit 7 of the separator 10 lies on top of the fuel gas conduit 8 of the separator 11. In FIG. 9, the oxidant gas conduit 7 on the separator 10 is displaced by one conduit width from the fuel gas conduit 8 on the separator 11. In both FIGS. 8 and 9, the two separators 10 and 11 contact each other in a hatched metal sealing portion 12, whereby electrical contact is obtained between the two separators 10 and 11. The sealing portion 12 also separates the coolant flow paths.
With either separator configuration as shown in FIGS. 8 and 9, uniform coolant flow and thus uniform cooling in the unit cell is difficult. In the arrangement shown in FIG. 8, uniform coolant flow is impeded by the combed-tooth, blind alley structure of the coolant flow path. In the arrangement shown in FIG. 9, although blind alleys are eliminated, uniform coolant flow is impeded by barriers caused by the convex portions next to the gas conduits 7 and 8.
In order to uniformly cool the electrodes in the electrode plane, a conventional cell structure interposes a conductive member, e.g., a metal plate, a metal mesh or a conductive carbon, between the anode side separator and the cathode side separator such that a flow path is available for the coolant. However, the additional conductive member has the undesired effects of increasing the fuel cell thickness, impairing cell performance due to contact resistance between the conductive member and the separators, and increasing manufacturing costs.
In view of the foregoing, an object of the present invention is to obviate the aforementioned shortcomings of conventional solid polymer electrolyte fuel cells.
A further object of the present invention is to provide a solid polymer electrolyte fuel cell wherein cell performance is not impaired by the condensation and liquefaction of the moisture in the reactant gases flowing through the unit cell.
Another object of the present invention is to provide a solid polymer electrolyte fuel cell characterized by high cell performance and uniform cooling in the electrode planes of the unit cells without employing additional elements, e.g., an additional conductive member.
Still another object of the present invention is to provide a compact solid polymer electrolyte fuel cell that can be manufactured at low cost.